In order to achieve both weight reduction and collision safety of automobiles, steel materials used for skeleton components have recently been required to be increased in strength.
However, in adoption of high-strength steel sheets, there is a concern of delayed fracture, which causes a hindrance to increasing the strength of the steel sheets for automobiles. The delayed fracture is a phenomenon that steel suffers brittle fracture after the elapse of a predetermined time in a state where a static load is applied, and is considered to be caused by hydrogen intruded into the steel. It has also been reported that when plastic strain is introduced into a steel sheet, delayed fracture is enhanced. Since large plastic strain is introduced into a cut end formed by shearing of a thin steel sheet, properties thereof are inferior, and the delayed fracture is considered to be liable to occur at the cut end in the thin steel sheet. When the delayed fracture occurs at the cut end in actual use environment to grow to a large crack, the member strength is degraded, which may lead to a serious accident.
It has therefore been eagerly desired to provide a steel sheet that has a high strength and is excellent in resistance to delayed fracture at a cut end thereof. More specifically, a steel sheet having a tensile strength of 1470 MPa or more and excellent resistance to delayed fracture at the cut end under actual environment has been required.
Conventionally, in order to increase the resistance to delayed fracture of high-strength steel sheets, many techniques using hydrogen trap sites have been proposed.
For example, Patent Literature 1 discloses a technique of increasing hydrogen trapping ability by dispersing an oxide in a steel sheet surface layer or a plated layer of a plated steel sheet, for the purpose of improving the resistance to delayed fracture at a welded part.
Further, Patent Literature 2 discloses a technique of utilizing a V-based carbide or the like as hydrogen trap sites, for the purpose of improving the resistance to delayed fracture after molding.
However, since the cut end of the steel sheet is a part to which extremely large deformation is added, the trap sites which trap hydrogen in an interface with a matrix, such as the oxides and carbides proposed in Patent literatures 1 and 2 described above, are changed in an interface structure by the large deformation to cause a problem that it becomes impossible to exhibit sufficient hydrogen trapping ability after cutting.
In addition, Patent Literature 3 discloses a technique of utilizing lath-shaped residual austenite as the hydrogen trap sites, for the purpose of improving the resistance to delayed fracture in punched parts.
Since residual austenite traps hydrogen in the inside thereof, even when the interface structure is changed with deformation caused by cutting, the hydrogen trapping activity is not lost thereby. However, since ordinary residual austenite is transformed to martensite by strain induced transformation when large deformation is added thereto, there is a problem that the hydrogen trapping ability is also degraded at the cut end with the large deformation.